On Thu, 21 Sep 2023 17:59:38 -0400 Eric DeVolder <eric.devol...@oracle.com> wrote:
> Scaled up testing has revealed that the kexec_trylock() > implementation leads to failures within the crash hotplug > infrastructure due to the inability to acquire the lock, > specifically the message: > > crash hp: kexec_trylock() failed, elfcorehdr may be inaccurate > > When hotplug events occur, the crash hotplug infrastructure first > attempts to obtain the lock via the kexec_trylock(). However, the > implementation either acquires the lock, or fails and returns; there > is no waiting on the lock. Here is the comment/explanation from > kernel/kexec_internal.h:kexec_trylock(): > > * Whatever is used to serialize accesses to the kexec_crash_image needs to be > * NMI safe, as __crash_kexec() can happen during nmi_panic(), so here we use > a > * "simple" atomic variable that is acquired with a cmpxchg(). > > While this in theory can happen for either CPU or memory hoptlug, > this problem is most prone to occur for memory hotplug. > > When memory is hot plugged, the memory is converted into smaller > 128MiB memblocks (typically). As each memblock is processed, a > kernel thread and a udev event thread are created. The udev thread > tries for the lock via the reading of the sysfs node > /sys/devices/system/memory/crash_hotplug node, and the kernel > worker thread tries for the lock upon entering the crash hotplug > infrastructure. > > These threads then compete for the kexec lock. > > For example, a 1GiB DIMM is converted into 8 memblocks, each > spawning two threads for a total of 16 threads that create a small > "swarm" all trying to acquire the lock. The larger the DIMM, the > more the memblocks and the larger the swarm. > > At the root of the problem is the atomic lock behind kexec_trylock(); > it works well for low lock traffic; ie loading/unloading a capture > kernel, things that happen basically once. But with the introduction > of crash hotplug, the traffic through the lock increases significantly, > and more importantly in bursts occurring at roughly the same time. Thus > there is a need to wait on the lock. > > A possible workaround is to simply retry the lock, say up to N times. > There is, of course, the problem of determining a value of N that works for > all implementations, and for all the other call sites of kexec_trylock(). > Not ideal. > > The design decision to use the atomic lock is described in the comment > from kexec_internal.h, cited above. However, examining the code of > __crash_kexec(): > > if (kexec_trylock()) { > if (kexec_crash_image) { > ... > } > kexec_unlock(); > } > > reveals that the use of kexec_trylock() here is actually a "best effort" > due to the atomic lock. This atomic lock, prior to crash hotplug, > would almost always be assured (another kexec syscall could hold the lock > and prevent this, but that is about it). > > So at the point where the capture kernel would be invoked, if the lock > is not obtained, then kdump doesn't occur. > > It is possible to instead use a mutex with proper waiting, and utilize > mutex_trylock() as the "best effort" in __crash_kexec(). The use of a > mutex then avoids all the lock acquisition problems that were revealed > by the crash hotplug activity. > > Convert the atomic lock to a mutex. > > ... > > --- a/kernel/kexec_core.c > +++ b/kernel/kexec_core.c > @@ -47,7 +47,7 @@ > #include <crypto/hash.h> > #include "kexec_internal.h" > > -atomic_t __kexec_lock = ATOMIC_INIT(0); > +DEFINE_MUTEX(__kexec_lock); > > /* Flag to indicate we are going to kexec a new kernel */ > bool kexec_in_progress = false; > @@ -1057,7 +1057,7 @@ void __noclone __crash_kexec(struct pt_regs *regs) > * of memory the xchg(&kexec_crash_image) would be > * sufficient. But since I reuse the memory... > */ > - if (kexec_trylock()) { > + if (mutex_trylock(&__kexec_lock)) { > if (kexec_crash_image) { > struct pt_regs fixed_regs; What's happening here? If someone else held the lock we silently fail to run the kexec? Shouldn't we at least alert the user to what just happened? _______________________________________________ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec