On Wed, 20 May 2020 22:39:11 +0530 Vinay Kumar Yadav wrote:
> On 5/20/2020 12:46 AM, David Miller wrote:
> > From: Vinay Kumar Yadav <vinay.ya...@chelsio.com>
> > Date: Tue, 19 May 2020 13:13:27 +0530
> >
> >> + spin_lock_bh(&ctx->encrypt_compl_lock);
> >> + pending = atomic_read(&ctx->encrypt_pending);
> >> + spin_unlock_bh(&ctx->encrypt_compl_lock);
> > The sequence:
> >
> > lock();
> > x = p->y;
> > unlock();
> >
> > Does not fix anything, and is superfluous locking.
> >
> > The value of p->y can change right after the unlock() call, so you
> > aren't protecting the atomic'ness of the read and test sequence
> > because the test is outside of the lock.
>
> Here, by using lock I want to achieve atomicity of following statements.
>
> pending = atomic_dec_return(&ctx->decrypt_pending);
> if (!pending && READ_ONCE(ctx->async_notify))
> complete(&ctx->async_wait.completion);
>
> means, don't want to read (atomic_read(&ctx->decrypt_pending))
> in middle of two statements
>
> atomic_dec_return(&ctx->decrypt_pending);
> and
> complete(&ctx->async_wait.completion);
>
> Why am I protecting only read, not test ?
Protecting code, not data, is rarely correct, though.
> complete() is called only if pending == 0
> if we read atomic_read(&ctx->decrypt_pending) = 0
> that means complete() is already called and its okay to
> initialize completion (reinit_completion(&ctx->async_wait.completion))
>
> if we read atomic_read(&ctx->decrypt_pending) as non zero that means:
> 1- complete() is going to be called or
> 2- complete() already called (if we read atomic_read(&ctx->decrypt_pending)
> == 1, then complete() is called just after unlock())
> for both scenario its okay to go into wait (crypto_wait_req(-EINPROGRESS,
> &ctx->async_wait))
First of all thanks for the fix, this completion code is unnecessarily
complex and brittle if you ask me.
That said I don't think your fix is 100%.
Consider this scenario:
# 1. writer queues first record on CPU0
# 2. encrypt completes on CPU1
pending = atomic_dec_return(&ctx->decrypt_pending);
# pending is 0
# IRQ comes and CPU1 goes off to do something else with spin lock held
# writer proceeds to encrypt next record on CPU0
# writer is done, enters wait
smp_store_mb(ctx->async_notify, true);
# Now CPU1 is back from the interrupt, does the check
if (!pending && READ_ONCE(ctx->async_notify))
complete(&ctx->async_wait.completion);
# and it completes the wait, even though the atomic decrypt_pending was
# bumped back to 1
You need to hold the lock around the async_notify false -> true
transition as well. The store no longer needs to have a barrier.
For async_notify true -> false transitions please add a comment
saying that there can be no concurrent accesses, since we have no
pending crypt operations.
Another way to solve this would be to add a large value to the pending
counter to indicate that there is a waiter:
if (atomic_add_and_fetch(&decrypt_pending, 1000) > 1000)
wait();
else
reinit();
atomic_sub(decrypt_pending, 1000)
completion:
if (atomic_dec_return(&decrypt_pending) == 1000)
complete()
