Messages in this thread | | | From | John Ogness <> | Subject | Re: devkmsg: was [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog_lock | Date | Sun, 06 Dec 2020 21:57:46 +0106 |
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On 2020-12-04, Petr Mladek <pmladek@suse.com> wrote: >> Since the ringbuffer is lockless, there is no need for it to be >> protected by @logbuf_lock. Remove @logbuf_lock. > > It might make sense to split also this patch into few more pieces that > would remove the lock from a particular interface.
OK.
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c >> index e9018c4e1b66..7385101210be 100644 >> --- a/kernel/printk/printk.c >> +++ b/kernel/printk/printk.c >> @@ -785,7 +749,6 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence) >> if (offset) >> return -ESPIPE; >> >> - logbuf_lock_irq(); > > user->seq manipulation is not longer safe from the atomicity point of > view. > > One solution would be to use atomic variable in struct devkmsg_user(). > Another solution would be to synchronize it with user->lock like we do > in devkmsg_read(). > > user->lock looks like an overhead. But it actually would make sense to > prevent seek in the middle of a read.
I would prefer using atomic64_t. Using user->lock could introduce some wacky regression.
>> switch (whence) { >> case SEEK_SET: >> /* the first record */ >> @@ -820,7 +782,6 @@ static __poll_t devkmsg_poll(struct file *file, poll_table *wait) >> >> poll_wait(file, &log_wait, wait); >> >> - logbuf_lock_irq(); >> if (prb_read_valid(prb, user->seq, NULL)) { > > Same here. The atomicity of user->seq read/write is not guaranteed.
Right.
John Ogness
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