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Date: Wed, 26 Jun 2019 18:47:09 -0400 From: "Theodore Ts'o" <tytso@....edu> To: syzbot <syzbot+4bfbbf28a2e50ab07368@...kaller.appspotmail.com>, adilger.kernel@...ger.ca, davem@...emloft.net, eladr@...lanox.com, idosch@...lanox.com, jiri@...lanox.com, john.stultz@...aro.org, linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, syzkaller-bugs@...glegroups.com, tglx@...utronix.de Subject: Re: INFO: rcu detected stall in ext4_write_checks More details about what is going on. First, it requires root, because one of that is required is using sched_setattr (which is enough to shoot yourself in the foot): sched_setattr(0, {size=0, sched_policy=0x6 /* SCHED_??? */, sched_flags=0, sched_nice=0, sched_priority=0, sched_runtime=2251799813724439, sched_deadline=4611686018427453437, sched_period=0}, 0) = 0 This is setting the scheduler policy to be SCHED_DEADLINE, with a runtime parameter of 2251799.813724439 seconds (or 26 days) and a deadline of 4611686018.427453437 seconds (or 146 *years*). This means a particular kernel thread can run for up to 26 **days** before it is scheduled away, and if a kernel reads gets woken up or sent a signal, no worries, it will wake up roughly seven times the interval that Rip Van Winkle spent snoozing in a cave in the Catskill Mountains (in Washington Irving's short story). We then kick off a half-dozen threads all running: sendfile(fd, fd, &pos, 0x8080fffffffe); (and since count is a ridiculously large number, this gets cut down to): sendfile(fd, fd, &pos, 2147479552); Is it any wonder that we are seeing RCU stalls? :-) - Ted
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