On 4/7/21 9:57 AM, John Garry wrote:
On 06/04/2021 22:49, Bart Van Assche wrote:
Since in the next patch knowledge is required of whether or not it is
allowed to sleep inside the tag iteration functions, pass this context
information to the tag iteration functions. I have reviewed all
callers of
tag iteration functions to verify these annotations by starting from the
output of the following grep command:
git grep -nHE 'blk_mq_(all_tag|tagset_busy)_iter'
My conclusions from that analysis are as follows:
- Sleeping is allowed in the blk-mq-debugfs code that iterates over tags.
- Since the blk_mq_tagset_busy_iter() calls in the mtip32xx driver are
preceded by a function that sleeps (blk_mq_quiesce_queue()),
sleeping is
safe in the context of the blk_mq_tagset_busy_iter() calls.
- The same reasoning also applies to the nbd driver.
- All blk_mq_tagset_busy_iter() calls in the NVMe drivers are followed
by a
call to a function that sleeps so sleeping inside
blk_mq_tagset_busy_iter()
when called from the NVMe driver is fine.
Hi Bart,
- scsi_host_busy(), scsi_host_complete_all_commands() and
scsi_host_busy_iter() are used by multiple SCSI LLDs so analyzing
whether
or not these functions may sleep is hard. Instead of performing that
analysis, make it safe to call these functions from atomic context.
Please help me understand this solution. The background is that we are
unsure if the SCSI iters callback functions may sleep. So we use the
blk_mq_all_tag_iter_atomic() iter, which tells us that we must not
sleep. And internally, it uses rcu read lock protection mechanism, which
relies on not sleeping. So it seems that we're making the SCSI iter
functions being safe in atomic context, and, as such, rely on the iter
callbacks not to sleep.
But if we call the SCSI iter function from non-atomic context and the
iter callback may sleep, then that is a problem, right? We're still
using rcu.
Hi John,
Please take a look at the output of the following grep command:
git grep -nHEw 'blk_mq_tagset_busy_iter|scsi_host_busy_iter'\ drivers/scsi
Do you agree with me that it is safe to call all the callback functions
passed to blk_mq_tagset_busy_iter() and scsi_host_busy_iter() from an
atomic context?
Thanks,
Bart.
