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 eGI8GIEC 0obv8r/JoHwmIRPBufbvXadisEf6dtSzHKUXtcKK+vYku6NUmzGhLtyV5zqIAwuJbqP/UsHa8XTvzwI3DGlXB+xCMx4rSDk2/E9dTcOtFpRyZKL0WLJjci3gQ8/GTr6ILVfoFucDL90UQF47JWmnVxU8XRhk1/YjW3nKEStPz66DMV9c2n5WzDifLQQ0d9tQAFyuq2h0m/jk6pHPv3fiwmWb10UcJwZ/xg4oGAv2XbXX6llZ8Ap7qgfl/Rhzf1WD2Nk4xujoJdUE9QYPv2x3X660yQPJoiDhFPMpfqOnQ0V96zFJt2graHV3wsaj+tsgTGEdvhgwYXdZADKmIlKUb1KJILAC1+2KsNHxV0v/KQYFQN/jbSnq8IgC+qIn7sXrYbLdtTmBxcwdb9z+MfJPbcQDrp2oRBwF5OePZMTSY65avtrOR9MYihAu17CnFySfUHhFFKC83GRo3NCyz4DLlkYjJLhnl8/zoBepKN2BvmryF2RPtYAElokQyIK2RMeI7+d9m+e1pauPGDYpM+swbtG3KsQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/24/26 17:03, Oleg Nesterov wrote: > On 02/24, Pavel Tikhomirov wrote: >> >> On 2/24/26 13:09, Oleg Nesterov wrote: >>> On 02/23, Pavel Tikhomirov wrote: >>>> >>>> To avoid possible problems related to cpu/compiler optimizations around >>>> ->child_reaper, let's use WRITE_ONCE (additional to task_list lock) >>>> everywhere we write it and use READ_ONCE everywhere we read it without >>>> explicit lock. >>> >>> Yes, this is what I meant... but I can never recall if READ_ONCE() alone >>> is enough to make KCSAN happy... >> >> AFAICS this should be fine for memory safety of accesses to ->child_reaper. >> I would love if someone more experienced in this area would confirm. > > __READ_ONCE() uses volatile cast, DEFINE_TSAN_VOLATILE_READ_WRITE() > will pass KCSAN_ACCESS_ATOMIC to check_access(), so it seems that > READ_ONCE() should be enough... > > But I am not sure, I don't really understand this code. > >>> I won't insist, but I think it would be better to do this in a separate >>> change for documenation purposes and for discussion. >> >> Ok, will do. It will be a bit ugly as I will first add READ_ONCE to the >> pidns_for_children_get() and then remove the hunk with it in the next patch. > > Agreed, this is ugly. I almost regret I mentioned _ONCE() in the previous > discussions, I only tried to "nack" another read_lock(tasklist). Heh, I missed that in pidns_for_children_get() we have read under tasklist lock, so I don't need to add _ONCE there so it should not be that ugly =). I will test and send v3 with prep patches soon. > > So lets avoid a separate change and WRITE_ONCE()'s in copy_process/find_child_reaper, > we can add them later if KCSAN complains, they are not needed for correctness. > > But up to you, I am fine either way. I also noticed that https://docs.kernel.org/dev-tools/kcsan.html#c.ASSERT_EXCLUSIVE_WRITER asks to add ASSERT_EXCLUSIVE_WRITER, so I will try to accommodate that. > >>> Perhaps something like the preparational patch below makes sense ? Not >>> sure this is actually better... >> >> This looks more universal at least, as instead of two checks we have one in one >> place. My only concern of putting the check where I put it was to avoid extra >> idr_alloc_cyclic() + idr_remove(), if we already know we don't need it. But it's >> only in last pid_namespace we can have ->child_reaper unset so we do alloc/remove >> for all other namespaces anyway in error case, should not be a big deal. > > Yes... > >> I will add the preparation patches: for below patch and related to _ONCE. > > Again, up to you. But either way it would be nice to have a comment or at > least a note in the changelog to explain that this is also needed to avoid > the race between alloc_pid() + fail and another alloc_pid(). This is subtle. Yes I will try to emphasize this. > > Oleg. > Thank you! -- Best regards, Pavel Tikhomirov Senior Software Developer, Virtuozzo.