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McKenney [230906 13:24]: > On Wed, Sep 06, 2023 at 11:23:25AM -0400, Liam R. Howlett wrote: > > (Adding Paul & Shanker to Cc list.. please see below for why) > > > > Apologies on the late response, I was away and have been struggling to > > get a working PPC32 test environment. > > > > * Geert Uytterhoeven [230829 12:42]: > > > Hi Liam, > > > > > > On Fri, 18 Aug 2023, Liam R. Howlett wrote: > > > > The current implementation of append may cause duplicate data and/or > > > > incorrect ranges to be returned to a reader during an update. Although > > > > this has not been reported or seen, disable the append write operation > > > > while the tree is in rcu mode out of an abundance of caution. > > > > ... > > > > > > > > Fixes: 54a611b60590 ("Maple Tree: add new data structure") > > > > Cc: stable@vger.kernel.org > > > > Signed-off-by: Liam R. Howlett > > > > > > Thanks for your patch, which is now commit cfeb6ae8bcb96ccf > > > ("maple_tree: disable mas_wr_append() when other readers are > > > possible") in v6.5, and is being backported to stable. > > > > > > On Renesas RZ/A1 and RZ/A2 (single-core Cortex-A9), this causes the > > > following warning: > > > > > > clocksource: timer@e803b000: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 28958491609 ns > > > sched_clock: 32 bits at 66MHz, resolution 15ns, wraps every 32537631224ns > > > /soc/timer@e803b000: used for clocksource > > > /soc/timer@e803c000: used for clock events > > > +------------[ cut here ]------------ > > > +WARNING: CPU: 0 PID: 0 at init/main.c:992 start_kernel+0x2f0/0x480 > > > +Interrupts were enabled early > > > > Note that the maple tree is involved in tracking the interrupts, see > > kernel/irq/irqdesc.c irq_insert_desc(), etc. > > > > > +CPU: 0 PID: 0 Comm: swapper Not tainted 6.5.0-rza2mevb-10197-g99b80d6b92b5 #237 > > > > I cannot find commit id 99b80d6b92b5. > > > > > +Hardware name: Generic R7S9210 (Flattened Device Tree) > > > + unwind_backtrace from show_stack+0x10/0x14 > > > + show_stack from dump_stack_lvl+0x24/0x3c > > > + dump_stack_lvl from __warn+0x74/0xb8 > > > + __warn from warn_slowpath_fmt+0x78/0xb0 > > > + warn_slowpath_fmt from start_kernel+0x2f0/0x480 > > > + start_kernel from 0x0 > > > +---[ end trace 0000000000000000 ]--- > > > Console: colour dummy device 80x30 > > > printk: console [tty0] enabled > > > Calibrating delay loop (skipped) preset value.. 1056.00 BogoMIPS (lpj=5280000) > > > > > > Reverting this commit fixes the issue. > > > > I have set up testing with qemu for powerpc 32b, and reverting this > > patch does not fix it for me. Did you revert the patch or bisect to the > > issue? > > > > It also happens on 0e0e9bd5f7b9 (I ran git checkout cfeb6ae8bcb96ccf^ to > > get the commit immediately before cfeb6ae8bcb96ccf). My qemu command is > > as follows: > > > > qemu-system-ppc -L pc-bios -boot c -prom-env "boot-device=hd:,\yaboot" > > -prom-env "boot-args=conf=hd:,\yaboot.conf" -M mac99,via=pmu -m 2048 > > -hda powerpc32.img -nographic -kernel > > > > > > > > > > RCU-related configs: > > > > > > $ grep RCU .config > > > # RCU Subsystem > > > CONFIG_TINY_RCU=y > > > # CONFIG_RCU_EXPERT is not set > > > CONFIG_TINY_SRCU=y > > > # end of RCU Subsystem > > > # RCU Debugging > > > # CONFIG_RCU_SCALE_TEST is not set > > > # CONFIG_RCU_TORTURE_TEST is not set > > > # CONFIG_RCU_REF_SCALE_TEST is not set > > > # CONFIG_RCU_TRACE is not set > > > # CONFIG_RCU_EQS_DEBUG is not set > > > # end of RCU Debugging > > > > I used the configuration from debian 8 and ran 'make oldconfig' to build > > my kernel. I have attached the configuration. > > > > > > > > CONFIG_MAPLE_RCU_DISABLED is not defined (and should BTW be renamed, > > > as CONFIG_* is reserved for kernel configuration options). > > > > Thanks, I'll add it to my list of work. > > > > > > > > I do not see this issue on any other platform > > > (arm/arm64/risc-v/mips/sh/m68k), several of them use the same > > > RCU configuration. > > > > > > Do you have a clue? > > > > It appears to be something to do with struct maple_tree sparse_irqs. If > > you drop the rcu flag from that maple tree, then my configuration boots > > without the warning. > > > > I *think* this is because we will reuse a lot more nodes. And I *think* > > the rcu flag is not needed, since there is a comment about reading the > > tree being protected by the mutex sparse_irq_lock within the > > kernel/irq/irqdesc.c file. Shanker, can you comment on that? > > > > I wonder if there is a limit to the number of RCU free events before > > something is triggered to flush them out which could trigger IRQ > > enabling? Paul, could this be the case? > > Are you asking if call_rcu() will re-enable interrupts in the following > use case? > > local_irq_disable(); > call_rcu(&p->rh, my_cb_func); > local_irq_enable(); > > If so, the answer is "no" (and yes, there might be a bug, but then again > I just double-checked). However, if interrupts are enabled across a > call_rcu() invocation, it will do some additional work to encourage > the grace period. Even if interrupts are disabled across a call_rcu() > invocation, it will still do either a raise_softirq() or a wakeup, > depending on configuration, to encourage the grace period. And in > the case of call_rcu() from an interrupt handler, that raise_softirq() > could result in the RCU_SOFTIRQ handler running upon return from that > interrupt, and if there are a great many callbacks ready to invoke, > this RCU_SOFTIRQ handler might take quite some time. Plus that > handler could itself be interrupted. > > If long-running RCU_SOFTIRQ handlers are a problem, you can boot with > rcutree.use_softirq=0, which puts that handler into a kthread, which > in turn give the scheduler more control. However, this will also turn > a lightweight raise_softirq() into a rather heavier weight wakeup. > Choose wisely! ;-) > > Or am I missing your point? > This is very early in the boot sequence when interrupts have not been enabled. What we are seeing is a WARN_ON() that is triggered by interrupts being enabled before they should be enabled. I was wondering if, for example, I called call_rcu() a lot *before* interrupts were enabled, that something could trigger that would either enable interrupts or indicate the task needs rescheduling? Specifically the rescheduling part is suspect. I tracked down the call to a mutex_lock() which calls cond_resched(), so could rcu be 'encouraging' the rcu window by a reschedule request? Thanks, Liam