From: Phil Hord <firstname.lastname@example.org> To: Matheus Tavares <email@example.com> Cc: Git <firstname.lastname@example.org>, Derrick Stolee <email@example.com>, Jonathan Tan <firstname.lastname@example.org>, Stefan Beller <email@example.com>, firstname.lastname@example.org Subject: Re: [PATCH v2 1/2] packfile: fix race condition on unpack_entry() Date: Fri, 2 Oct 2020 13:06:46 -0700 [thread overview] Message-ID: <CABURp0ovz0G-mYDv+CmL_pSu09WDJTxNQodpnyT3MrYjKjusEw@mail.gmail.com> (raw) In-Reply-To: <email@example.com> On Mon, Sep 28, 2020 at 5:02 PM Matheus Tavares <firstname.lastname@example.org> wrote: > > The third phase of unpack_entry() performs the following sequence in a > loop, until all the deltas enumerated in phase one are applied and the > entry is fully reconstructed: > > 1. Add the current base entry to the delta base cache > 2. Unpack the next delta > 3. Patch the unpacked delta on top of the base > > When the optional object reading lock is enabled, the above steps will > be performed while holding the lock. However, step 2. momentarily > releases it so that inflation can be performed in parallel for increased > performance. Because the `base` buffer inserted in the cache at 1. is > not duplicated, another thread can potentially free() it while the lock > is released at 2. (e.g. when there is no space left in the cache to > insert another entry). In this case, the later attempt to dereference > `base` at 3. will cause a segmentation fault. This problem was observed > during a multithreaded git-grep execution on a repository with large > objects. > > To fix the race condition (and later segmentation fault), let's reorder > the aforementioned steps so that `base` is only added to the cache at > the end. This will prevent the buffer from being released by another > thread while it is still in use. An alternative solution which would not > require the reordering would be to duplicate `base` before inserting it > in the cache. However, as Phil Hord mentioned, memcpy()'ing large bases > can negatively affect performance: in his experiments, this alternative > approach slowed git-grep down by 10% to 20%. > > Reported-by: Phil Hord <email@example.com> > Signed-off-by: Matheus Tavares <firstname.lastname@example.org> > --- Thanks for looking after this so quickly. This all looks good to me, and I confirmed it does fix the problems I was seeing. Phil
next prev parent reply other threads:[~2020-10-02 20:07 UTC|newest] Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-09-25 2:36 RFC - concurrency causes segfault in git grep since 2.26.0 Phil Hord 2020-09-25 5:52 ` Matheus Tavares 2020-09-25 19:53 ` Phil Hord 2020-09-28 16:50 ` [PATCH 0/2] Fix race condition and memory leak in delta base cache Matheus Tavares 2020-09-28 16:50 ` [PATCH 1/2] packfile: fix race condition on unpack_entry() Matheus Tavares 2020-09-28 18:05 ` Junio C Hamano 2020-09-28 16:50 ` [PATCH 2/2] packfile: fix memory leak in add_delta_base_cache() Matheus Tavares 2020-09-28 18:22 ` Junio C Hamano 2020-09-29 0:01 ` [PATCH v2 0/2] Fix race condition and memory leak in delta base cache Matheus Tavares 2020-09-29 0:01 ` [PATCH v2 1/2] packfile: fix race condition on unpack_entry() Matheus Tavares 2020-10-02 20:06 ` Phil Hord [this message] 2020-09-29 0:01 ` [PATCH v2 2/2] packfile: fix memory leak in add_delta_base_cache() Matheus Tavares
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