From: John Ogness <john.ogness@linutronix.de>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t
Date: Sun, 06 Dec 2020 21:29:59 +0106 [thread overview]
Message-ID: <875z5eof8g.fsf@jogness.linutronix.de> (raw)
In-Reply-To: <X8n9a2DWUFE/giyB@alley>
On 2020-12-04, Petr Mladek <pmladek@suse.com> wrote:
> On Tue 2020-12-01 21:59:40, John Ogness wrote:
>> Currently @clear_seq access is protected by @logbuf_lock. Once
>> @logbuf_lock is removed some other form of synchronization will be
>> required. Change the type of @clear_seq to atomic64_t to provide the
>> synchronization.
>>
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>> index fc5e3a7d6d89..e9018c4e1b66 100644
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -3412,7 +3418,7 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
>> */
>> void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
>> {
>> - dumper->cur_seq = clear_seq;
>> + dumper->cur_seq = atomic64_read(&clear_seq);
>
> Sigh, atomic64_read() uses a spin lock in the generic implementation
> that is used on some architectures.
>
> Hmm, this seems to be the only location where the lock must not be
> used.
Yes, and it is read-only access. Perhaps atomic64_t is the wrong thing
to use here. We could use a seqcount_latch and a shadow variable so that
if a writer has been preempted, we can use the previous value. (Only
kmsg_dump would need to use the lockless variant to read the value.)
void clear_seq_set(u64 val)
{
spin_lock_irq(&clear_lock);
raw_write_seqcount_latch(&clear_latch);
clear_seq[0] = val;
raw_write_seqcount_latch(&clear_latch);
clear_seq[1] = val;
spin_unlock_irq(&clear_lock);
}
u64 clear_seq_get_nolock(void)
{
unsigned int seq, idx;
u64 val;
do {
seq = raw_read_seqcount_latch(&clear_latch);
idx = seq & 0x1;
val = clear_seq[idx];
} while (read_seqcount_latch_retry(&clear_latch, seq));
return val;
}
u64 clear_seq_get(void)
{
u64 val;
spin_lock_irq(&clear_lock);
val = clear_seq[0];
spin_unlock_irq(&clear_lock);
return val;
}
> Alternative solution would to always fallback to the first_seq on
> these architectures. Few people would complain when they see more
> messages. We could always improve it when it causes problems.
I am also OK with this solution.
John Ogness
next prev parent reply other threads:[~2020-12-06 20:24 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-01 20:53 [PATCH next v2 0/3] printk: remove logbuf_lock John Ogness
2020-12-01 20:53 ` [PATCH next v2 1/3] printk: inline log_output(),log_store() in vprintk_store() John Ogness
2020-12-03 15:57 ` Petr Mladek
2020-12-03 16:25 ` John Ogness
2020-12-04 6:13 ` Sergey Senozhatsky
2020-12-04 8:26 ` Petr Mladek
2020-12-01 20:53 ` [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t John Ogness
2020-12-04 9:12 ` Petr Mladek
2020-12-06 20:23 ` John Ogness [this message]
2020-12-07 9:34 ` Peter Zijlstra
2020-12-07 10:03 ` John Ogness
2020-12-07 12:56 ` Peter Zijlstra
2020-12-07 12:56 ` Petr Mladek
2020-12-07 16:46 ` David Laight
2020-12-08 20:34 ` Sergey Senozhatsky
2020-12-08 22:30 ` John Ogness
2020-12-09 1:04 ` Sergey Senozhatsky
2020-12-09 8:16 ` Peter Zijlstra
2020-12-09 9:22 ` Sergey Senozhatsky
2020-12-09 10:46 ` Sergey Senozhatsky
2020-12-09 11:00 ` Peter Zijlstra
2020-12-09 11:28 ` Sergey Senozhatsky
2020-12-09 12:29 ` Peter Zijlstra
2020-12-09 8:07 ` Peter Zijlstra
2020-12-01 20:53 ` [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog_lock John Ogness
2020-12-04 6:41 ` Sergey Senozhatsky
2020-12-06 20:44 ` John Ogness
2020-12-04 15:52 ` devkmsg: was " Petr Mladek
2020-12-06 20:51 ` John Ogness
2020-12-07 9:56 ` Petr Mladek
2020-12-04 15:57 ` syslog: was: " Petr Mladek
2020-12-06 21:06 ` John Ogness
2020-12-07 10:01 ` Petr Mladek
2020-12-04 16:10 ` recursion handling: " Petr Mladek
2020-12-05 4:25 ` Sergey Senozhatsky
2020-12-06 22:08 ` John Ogness
2020-12-05 9:41 ` Sergey Senozhatsky
2020-12-06 22:17 ` John Ogness
2020-12-06 21:44 ` John Ogness
2020-12-07 11:17 ` Petr Mladek
2020-12-04 16:15 ` vprintk_store: was: " Petr Mladek
2020-12-06 22:30 ` John Ogness
2020-12-07 12:46 ` Petr Mladek
2020-12-04 16:19 ` consoles: " Petr Mladek
2020-12-05 4:39 ` Sergey Senozhatsky
2020-12-07 9:50 ` Petr Mladek
2020-12-08 20:51 ` Sergey Senozhatsky
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