From: Gabriel Krisman Bertazi <krisman@collabora.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de, mingo@redhat.com, dvhart@infradead.org,
linux-kernel@vger.kernel.org, kernel@collabora.com,
Zebediah Figura <z.figura12@gmail.com>,
Steven Noonan <steven@valvesoftware.com>,
"Pierre-Loup A . Griffais" <pgriffais@valvesoftware.com>,
viro@zeniv.linux.org.uk, jannh@google.com
Subject: Re: [PATCH RFC 2/2] futex: Implement mechanism to wait on any of several futexes
Date: Tue, 06 Aug 2019 02:26:38 -0400 [thread overview]
Message-ID: <85imra6c81.fsf@collabora.com> (raw)
In-Reply-To: <20190731120600.GT31381@hirez.programming.kicks-ass.net> (Peter Zijlstra's message of "Wed, 31 Jul 2019 14:06:00 +0200")
Peter Zijlstra <peterz@infradead.org> writes:
>
>> +static int futex_wait_multiple(u32 __user *uaddr, unsigned int flags,
>> + u32 count, ktime_t *abs_time)
>> +{
>> + struct futex_wait_block *wb;
>> + struct restart_block *restart;
>> + int ret;
>> +
>> + if (!count)
>> + return -EINVAL;
>> +
>> + wb = kcalloc(count, sizeof(struct futex_wait_block), GFP_KERNEL);
>> + if (!wb)
>> + return -ENOMEM;
>> +
>> + if (copy_from_user(wb, uaddr,
>> + count * sizeof(struct futex_wait_block))) {
>> + ret = -EFAULT;
>> + goto out;
>> + }
>
> I'm thinking we can do away with this giant copy and do it one at a time
> from the other function, just extend the storage allocated there to
> store whatever values are still required later.
Hey Peter,
Thanks for your very detailed review. it is deeply appreciated. My
apologies for the style issues, I blindly trusted checkpatch.pl, when it
said it was ready for submission.
I'm not sure I get the suggestion here. If I understand the code
correctly, once we do it one at a time, we need to queue_me() each futex
and then drop the hb lock, before going to the next one. Once we go to
the next one, we need to call get_user_pages (and now copy_from_user),
both of which can sleep, and on return set the task state to
TASK_RUNNING. This opens a window where we can wake up the task but it
is not in the right sleeping state, which from the comment in
futex_wait_queue_me(), seems problematic. This is also the reason why I
wanted to split the key memory pin from the actual read in patch 1/2.
Did you consider this problem or is it not a problem for some reason?
What am I missing?
--
Gabriel Krisman Bertazi
next prev parent reply other threads:[~2019-08-06 6:26 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-30 22:06 [PATCH RFC 1/2] futex: Split key setup from key queue locking and read Gabriel Krisman Bertazi
2019-07-30 22:06 ` [PATCH RFC 2/2] futex: Implement mechanism to wait on any of several futexes Gabriel Krisman Bertazi
2019-07-31 12:06 ` Peter Zijlstra
2019-07-31 15:15 ` Zebediah Figura
2019-07-31 22:39 ` Thomas Gleixner
2019-07-31 23:02 ` Zebediah Figura
2019-08-06 6:26 ` Gabriel Krisman Bertazi [this message]
2019-08-06 10:13 ` Peter Zijlstra
2019-08-01 0:45 ` Thomas Gleixner
2019-08-01 1:22 ` Zebediah Figura
2019-08-01 1:32 ` Zebediah Figura
2019-08-01 1:42 ` Pierre-Loup A. Griffais
2019-07-31 23:33 ` [PATCH RFC 1/2] futex: Split key setup from key queue locking and read Thomas Gleixner
2019-08-01 0:07 ` Gabriel Krisman Bertazi
2019-08-01 0:22 ` Gabriel Krisman Bertazi
2019-08-01 0:41 ` Thomas Gleixner
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