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Biederman" , linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-rt-users@vger.kernel.org, Arnd Bergmann , Thomas Gleixner , Sebastian Andrzej Siewior , Juri Lelli , "Luis Claudio R. Goncalves" , Andrew Morton , Vivek Goyal Subject: Re: [PATCH v2] panic, kexec: Make __crash_kexec() NMI safe In-Reply-To: <20220629115539.GB12720@pathway.suse.cz> References: <20220620111520.1039685-1-vschneid@redhat.com> <87r13c7jyp.fsf@email.froward.int.ebiederm.org> <20220629115539.GB12720@pathway.suse.cz> Date: Wed, 29 Jun 2022 13:23:08 +0100 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29/06/22 13:55, Petr Mladek wrote: > On Tue 2022-06-28 18:33:08, Valentin Schneider wrote: >> >> 8c5a1cf0ad3a ("kexec: use a mutex for locking rather than xchg()") was >> straightforward enough because it turned >> >> if (xchg(&lock, 1)) >> return -EBUSY; >> >> into >> >> if (!mutex_trylock(&lock)) >> return -EBUSY; >> >> Now, most of the kexec_mutex uses are trylocks, except for: >> - crash_get_memory_size() >> - crash_shrink_memory() >> >> I really don't want to go down the route of turning those into cmpxchg >> try-loops, would it be acceptable to make those use trylocks (i.e. return >> -EBUSY if the cmpxchg fails)? > > IMHO, -EBUSY is acceptable for both crash_get_memory_size() > and crash_shrink_memory(). They are used in the sysfs interface. > >> Otherwise, we keep the mutexes for functions like those which go nowhere >> near an NMI. > > If we go this way then I would hide the locking into some wrappers, > like crash_kexec_trylock()/unlock() that would do both mutex > and xchg. The xchg part might be hidden in a separate wrapper > __crash_kexec_trylock()/unlock() or > crash_kexec_atomic_trylock()/unlock(). > Makes sense, thanks. I've started playing with the trylock/-EBUSY approach, I'll toss it out if I don't end up hating it. > Best Regards, > Petr