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From Ian Holsman <i...@cnet.com>
Subject Re: [PATCH 2] Re: [PATCH] mmap_setaside (with apr_mmap_dup)
Date Wed, 21 Nov 2001 17:18:44 GMT
Thanks Brian.
It's commited.

On Thu, 2001-11-15 at 20:35, Brian Pane wrote:
> Greg Stein wrote:
> [...]
> 
> >>I gave up trying to do full reference counting semantics for
> >>duplicates of apr_mmap_t, because I couldn't find a suitable
> >>pool to own the locks that would be required in a threaded
> >>implementation.
> >>
> >
> >Not sure what you mean here. I don't quite understand how threading comes
> >into play. apr_file_t and apr_mmap_t can't be used by multiple threads,
> >AFAIK. And I don't think they should either...
> >
> 
> When I tried to implement a traditional reference-counting solution
> (all apr_mmap_t's duplicated from the same original share a reference
> count, and the mmap'ed segment is destroyed only when the refcount
> reaches zero), I ran into threading problems with mod_file_cache.
> It's possible for two threads to be doing a dup of the same
> cached apr_mmap_t at the same time--if they're both delivering the
> same statically cached file, and they both end up having to call
> mmap_setaside.  To avoid the race condition here, we'd need a lock
> around the reference count. The lock API requires that we create the
> lock from a pool, but there isn't a really suitable pool from which
> to allocate the lock, given that its lifetime isn't necessarily tied
> to that of a request or connection.
> 
> Fortunately, this whole problem went away when I switched from the
> reference-counted model to the transfer-of-ownership model.  The latter
> doesn't require any locking.
> 
> >>...
> >>--- srclib/apr/mmap/win32/mmap.c	2001/06/27 22:07:24	1.6
> >>+++ srclib/apr/mmap/win32/mmap.c	2001/11/15 05:45:58
> >>@@ -62,10 +62,15 @@
> >> 
> >> #if APR_HAS_MMAP
> >> 
> >>-static apr_status_t mmap_cleanup(void *themmap)
> >>+APR_DECLARE(apr_status_t) apr_mmap_cleanup(void *themmap)
> >> {
> >>
> >
> >Why is this APR_DECLARE'd now? And why the function rename?
> >
> This is an error left over from an earlier design approach that I tried
> and abandoned.  Thanks for catching that.  I've attached a new version
> of the patch that fixes it.
> 
> --Brian
> 
> 
> 
> ----
> 

> Index: srclib/apr/include/apr_mmap.h
> ===================================================================
> RCS file: /home/cvspublic/apr/include/apr_mmap.h,v
> retrieving revision 1.25
> diff -u -r1.25 apr_mmap.h
> --- srclib/apr/include/apr_mmap.h	2001/08/18 15:15:46	1.25
> +++ srclib/apr/include/apr_mmap.h	2001/11/16 04:22:14
> @@ -111,6 +111,8 @@
>      void *mm;
>      /** The amount of data in the mmap */
>      apr_size_t size;
> +    /** Whether this object is reponsible for doing the munmap */
> +    int is_owner;
>  };
>  
>  #if APR_HAS_MMAP || defined(DOXYGEN)
> @@ -134,6 +136,19 @@
>                                            apr_file_t *file, apr_off_t offset,
>                                            apr_size_t size, apr_int32_t flag,
>                                            apr_pool_t *cntxt);
> +
> +/**
> + * Duplicate the specified MMAP.
> + * @param new_mmap The structure to duplicate into. 
> + * @param old_mmap The file to duplicate.
> + * @param p The pool to use for the new file.
> + * @param transfer_ownership  Whether responsibility for destroying
> + *  the memory-mapped segment is transferred from old_mmap to new_mmap
> + */         
> +APR_DECLARE(apr_status_t) apr_mmap_dup(apr_mmap_t **new_mmap,
> +                                       apr_mmap_t *old_mmap,
> +                                       apr_pool_t *p,
> +                                       int transfer_ownership);
>  
>  /**
>   * Remove a mmap'ed.
> Index: srclib/apr/mmap/unix/mmap.c
> ===================================================================
> RCS file: /home/cvspublic/apr/mmap/unix/mmap.c,v
> retrieving revision 1.36
> diff -u -r1.36 mmap.c
> --- srclib/apr/mmap/unix/mmap.c	2001/08/02 04:26:53	1.36
> +++ srclib/apr/mmap/unix/mmap.c	2001/11/16 04:22:14
> @@ -83,6 +83,11 @@
>  {
>      apr_mmap_t *mm = themmap;
>      int rv;
> +
> +    if (!mm->is_owner) {
> +        return APR_SUCCESS;
> +    }
> +
>  #ifdef BEOS
>      rv = delete_area(mm->area);
>  
> @@ -159,10 +164,37 @@
>      (*new)->mm = mm;
>      (*new)->size = size;
>      (*new)->cntxt = cont;
> +    (*new)->is_owner = 1;
>  
>      /* register the cleanup... */
>      apr_pool_cleanup_register((*new)->cntxt, (void*)(*new), mmap_cleanup,
>               apr_pool_cleanup_null);
> +    return APR_SUCCESS;
> +}
> +
> +APR_DECLARE(apr_status_t) apr_mmap_dup(apr_mmap_t **new_mmap,
> +                                       apr_mmap_t *old_mmap,
> +                                       apr_pool_t *p,
> +                                       int transfer_ownership)
> +{
> +    *new_mmap = (apr_mmap_t *)apr_pmemdup(p, old_mmap, sizeof(apr_mmap_t));
> +    (*new_mmap)->cntxt = p;
> +
> +    /* The old_mmap can transfer ownership only if the old_mmap itself
> +     * is an owner of the mmap'ed segment.
> +     */
> +    if (old_mmap->is_owner) {
> +        if (transfer_ownership) {
> +            (*new_mmap)->is_owner = 1;
> +            old_mmap->is_owner = 0;
> +            apr_pool_cleanup_kill(old_mmap->cntxt, old_mmap, mmap_cleanup);
> +        }
> +        else {
> +            (*new_mmap)->is_owner = 0;
> +        }
> +        apr_pool_cleanup_register(p, *new_mmap, mmap_cleanup,
> +                                  apr_pool_cleanup_null);
> +    }
>      return APR_SUCCESS;
>  }
>  
> Index: srclib/apr/mmap/win32/mmap.c
> ===================================================================
> RCS file: /home/cvspublic/apr/mmap/win32/mmap.c,v
> retrieving revision 1.6
> diff -u -r1.6 mmap.c
> --- srclib/apr/mmap/win32/mmap.c	2001/06/27 22:07:24	1.6
> +++ srclib/apr/mmap/win32/mmap.c	2001/11/16 04:22:14
> @@ -66,6 +66,11 @@
>  {
>      apr_mmap_t *mm = themmap;
>      apr_status_t rv = 0;
> +
> +    if (!mm->is_owner) {
> +        return APR_SUCCESS;
> +    }
> +
>      if (mm->mv) {
>          if (!UnmapViewOfFile(mm->mv))
>          {
> @@ -155,10 +160,37 @@
>      (*new)->mm = (char*)((*new)->mv) + offset;
>      (*new)->size = size;
>      (*new)->cntxt = cont;
> +    (*new)->is_owner = 1;
>  
>      /* register the cleanup... */
>      apr_pool_cleanup_register((*new)->cntxt, (void*)(*new), mmap_cleanup,
>                           apr_pool_cleanup_null);
> +    return APR_SUCCESS;
> +}
> +
> +APR_DECLARE(apr_status_t) apr_mmap_dup(apr_mmap_t **new_mmap,
> +                                       apr_mmap_t *old_mmap,
> +                                       apr_pool_t *p,
> +                                       int transfer_ownership)
> +{
> +    *new_mmap = (apr_mmap_t *)apr_pmemdup(p, old_mmap, sizeof(apr_mmap_t));
> +    (*new_mmap)->cntxt = p;
> +
> +    /* The old_mmap can transfer ownership only if the old_mmap itself
> +     * is an owner of the mmap'ed segment.
> +     */
> +    if (old_mmap->is_owner) {
> +        if (transfer_ownership) {
> +            (*new_mmap)->is_owner = 1;
> +            old_mmap->is_owner = 0;
> +            apr_pool_cleanup_kill(old_mmap->cntxt, old_mmap, mmap_cleanup);
> +        }
> +        else {
> +            (*new_mmap)->is_owner = 0;
> +        }
> +        apr_pool_cleanup_register(p, *new_mmap, mmap_cleanup,
> +                                  apr_pool_cleanup_null);
> +    }
>      return APR_SUCCESS;
>  }
>  
> Index: srclib/apr-util/buckets/apr_buckets_mmap.c
> ===================================================================
> RCS file: /home/cvspublic/apr-util/buckets/apr_buckets_mmap.c,v
> retrieving revision 1.43
> diff -u -r1.43 apr_buckets_mmap.c
> --- srclib/apr-util/buckets/apr_buckets_mmap.c	2001/09/22 22:36:07	1.43
> +++ srclib/apr-util/buckets/apr_buckets_mmap.c	2001/11/16 04:22:14
> @@ -121,24 +121,18 @@
>  {
>      apr_bucket_mmap *m = data->data;
>      apr_mmap_t *mm = m->mmap;
> -    char *base;
> -    void *addr;
> +    apr_mmap_t *new_mm;
>      apr_status_t ok;
>  
>      if (apr_pool_is_ancestor(mm->cntxt, p)) {
>          return APR_SUCCESS;
>      }
>  
> -    ok = apr_mmap_offset(&addr, m->mmap, data->start);
> +    ok = apr_mmap_dup(&new_mm, mm, p, 1);
>      if (ok != APR_SUCCESS) {
>          return ok;
>      }
> -
> -    base = apr_palloc(p, data->length);
> -    memcpy(base, addr, data->length);
> -    data = apr_bucket_pool_make(data, base, data->length, p);
> -    mmap_destroy(m);
> -
> +    m->mmap = new_mm;
>      return APR_SUCCESS;
>  }
>  
> Index: modules/cache/mod_file_cache.c
> ===================================================================
> RCS file: /home/cvspublic/httpd-2.0/modules/cache/mod_file_cache.c,v
> retrieving revision 1.62
> diff -u -r1.62 mod_file_cache.c
> --- modules/cache/mod_file_cache.c	2001/08/11 04:04:12	1.62
> +++ modules/cache/mod_file_cache.c	2001/11/16 04:22:15
> @@ -333,8 +333,20 @@
>  #if APR_HAS_MMAP
>      apr_bucket *b;
>      apr_bucket_brigade *bb = apr_brigade_create(r->pool);
> +    apr_mmap_t *mm;
>  
> -    b = apr_bucket_mmap_create(file->mm, 0, (apr_size_t)file->finfo.size);
> +    /* Create a copy of the apr_mmap_t that's marked as
> +     * a "non-owner."  The reason for this is that the
> +     * mmap bucket setaside function might later try to
> +     * transfer ownership of the mmap from a request
> +     * pool to a parent pool.  For this particular mmap,
> +     * though, we want the cache to retain ownership so
> +     * that it never gets munmapped.  Thus we give the
> +     * bucket code a non-owner copy to work with.
> +     */
> +    apr_mmap_dup(&mm, file->mm, r->pool, 0);
> +
> +    b = apr_bucket_mmap_create(mm, 0, (apr_size_t)file->finfo.size);
>      APR_BRIGADE_INSERT_TAIL(bb, b);
>      b = apr_bucket_eos_create();
>      APR_BRIGADE_INSERT_TAIL(bb, b);
-- 
Ian Holsman          IanH@cnet.com
Performance Measurement & Analysis
CNET Networks   -   (415) 344-2608


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