Re: [PATCH 0/3] aio: ctx->dead cleanups

From: Al Viro
Date: Tue Jun 16 2015 - 20:39:40 EST

On Wed, Jun 17, 2015 at 01:04:14AM +0200, Oleg Nesterov wrote:
> Al, please help. We are trying to backport some aio fixes and I am
> absolutely confused by your b2edffdd912b "fix mremap() vs. ioctx_kill()
> race".
> Firstly, I simply can't understand what exactly it tries to fix. OK,
> aio_free_ring() can race with kill and we can remap the soon-to-be-killed
> ctx. So what? kill_ioctx() will the the correct (already re-mapped)
> ctx->mmap_base after it drops mm->ioctx_lock.

Huh? kill_ioctx() picks ctx->mmap_base and passes it to vm_munmap().
Which tries to grab mmap_sem, blocks for mremap() from another thread
and waits for it to drop mmap_sem. By that time ctx->mmap_base has
nothing whatsoever to the argument we'd passed to vm_munmap(). Sure,
it had been recalculated by aio_ring_remap(), but it's too late for
us - we'd already fetched the old value.
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