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[217.229.29.45]) by smtp.gmail.com with ESMTPSA id p1sm3138013wmi.9.2019.03.12.05.45.33 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 12 Mar 2019 05:45:33 -0700 (PDT) Date: Tue, 12 Mar 2019 13:45:32 +0100 From: Thierry Reding To: Yoshihiro Shimoda Cc: linux-pwm@vger.kernel.org, linux-renesas-soc@vger.kernel.org, Phong Hoang Subject: Re: [PATCH] pwm: Avoid deadlock warning when removing PWM device Message-ID: <20190312124532.GO31026@ulmo> References: <1552360594-21547-1-git-send-email-yoshihiro.shimoda.uh@renesas.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="vNrHrykRFvLVX6W3" Content-Disposition: inline In-Reply-To: <1552360594-21547-1-git-send-email-yoshihiro.shimoda.uh@renesas.com> User-Agent: Mutt/1.11.3 (2019-02-01) Sender: linux-renesas-soc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-renesas-soc@vger.kernel.org --vNrHrykRFvLVX6W3 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Mar 12, 2019 at 12:16:34PM +0900, Yoshihiro Shimoda wrote: > From: Phong Hoang >=20 > This patch fixes deadlock warning if removing PWM device > when CONFIG_PROVE_LOCKING is enabled. >=20 > This issue can be reproceduced by the following steps on > the R-Car H3 Salvator-X board if the backlight is disabled: >=20 > # cd /sys/class/pwm/pwmchip0 > # echo 0 > export > # ls > device export npwm power pwm0 subsystem uevent unexport > # cd device/driver > # ls > bind e6e31000.pwm uevent unbind > # echo e6e31000.pwm > unbind >=20 > [ 87.659974] =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > [ 87.666149] WARNING: possible circular locking dependency detected > [ 87.672327] 5.0.0 #7 Not tainted > [ 87.675549] ------------------------------------------------------ > [ 87.681723] bash/2986 is trying to acquire lock: > [ 87.686337] 000000005ea0e178 (kn->count#58){++++}, at: kernfs_remove_b= y_name_ns+0x50/0xa0 > [ 87.694528] > [ 87.694528] but task is already holding lock: > [ 87.700353] 000000006313b17c (pwm_lock){+.+.}, at: pwmchip_remove+0x28= /0x13c > [ 87.707405] > [ 87.707405] which lock already depends on the new lock. > [ 87.707405] > [ 87.715574] > [ 87.715574] the existing dependency chain (in reverse order) is: > [ 87.723048] > [ 87.723048] -> #1 (pwm_lock){+.+.}: > [ 87.728017] __mutex_lock+0x70/0x7e4 > [ 87.732108] mutex_lock_nested+0x1c/0x24 > [ 87.736547] pwm_request_from_chip.part.6+0x34/0x74 > [ 87.741940] pwm_request_from_chip+0x20/0x40 > [ 87.746725] export_store+0x6c/0x1f4 > [ 87.750820] dev_attr_store+0x18/0x28 > [ 87.754998] sysfs_kf_write+0x54/0x64 > [ 87.759175] kernfs_fop_write+0xe4/0x1e8 > [ 87.763615] __vfs_write+0x40/0x184 > [ 87.767619] vfs_write+0xa8/0x19c > [ 87.771448] ksys_write+0x58/0xbc > [ 87.775278] __arm64_sys_write+0x18/0x20 > [ 87.779721] el0_svc_common+0xd0/0x124 > [ 87.783986] el0_svc_compat_handler+0x1c/0x24 > [ 87.788858] el0_svc_compat+0x8/0x18 > [ 87.792947] > [ 87.792947] -> #0 (kn->count#58){++++}: > [ 87.798260] lock_acquire+0xc4/0x22c > [ 87.802353] __kernfs_remove+0x258/0x2c4 > [ 87.806790] kernfs_remove_by_name_ns+0x50/0xa0 > [ 87.811836] remove_files.isra.1+0x38/0x78 > [ 87.816447] sysfs_remove_group+0x48/0x98 > [ 87.820971] sysfs_remove_groups+0x34/0x4c > [ 87.825583] device_remove_attrs+0x6c/0x7c > [ 87.830197] device_del+0x11c/0x33c > [ 87.834201] device_unregister+0x14/0x2c > [ 87.838638] pwmchip_sysfs_unexport+0x40/0x4c > [ 87.843509] pwmchip_remove+0xf4/0x13c > [ 87.847773] rcar_pwm_remove+0x28/0x34 > [ 87.852039] platform_drv_remove+0x24/0x64 > [ 87.856651] device_release_driver_internal+0x18c/0x21c > [ 87.862391] device_release_driver+0x14/0x1c > [ 87.867175] unbind_store+0xe0/0x124 > [ 87.871265] drv_attr_store+0x20/0x30 > [ 87.875442] sysfs_kf_write+0x54/0x64 > [ 87.879618] kernfs_fop_write+0xe4/0x1e8 > [ 87.884055] __vfs_write+0x40/0x184 > [ 87.888057] vfs_write+0xa8/0x19c > [ 87.891887] ksys_write+0x58/0xbc > [ 87.895716] __arm64_sys_write+0x18/0x20 > [ 87.900154] el0_svc_common+0xd0/0x124 > [ 87.904417] el0_svc_compat_handler+0x1c/0x24 > [ 87.909289] el0_svc_compat+0x8/0x18 > [ 87.913378] > [ 87.913378] other info that might help us debug this: > [ 87.913378] > [ 87.921374] Possible unsafe locking scenario: > [ 87.921374] > [ 87.927286] CPU0 CPU1 > [ 87.931808] ---- ---- > [ 87.936331] lock(pwm_lock); > [ 87.939293] lock(kn->count#58); > [ 87.945120] lock(pwm_lock); > [ 87.950599] lock(kn->count#58); > [ 87.953908] > [ 87.953908] *** DEADLOCK *** > [ 87.953908] > [ 87.959821] 4 locks held by bash/2986: > [ 87.963563] #0: 00000000ace7bc30 (sb_writers#6){.+.+}, at: vfs_write+= 0x188/0x19c > [ 87.971044] #1: 00000000287991b2 (&of->mutex){+.+.}, at: kernfs_fop_w= rite+0xb4/0x1e8 > [ 87.978872] #2: 00000000f739d016 (&dev->mutex){....}, at: device_rele= ase_driver_internal+0x40/0x21c > [ 87.988001] #3: 000000006313b17c (pwm_lock){+.+.}, at: pwmchip_remove= +0x28/0x13c > [ 87.995481] > [ 87.995481] stack backtrace: > [ 87.999836] CPU: 0 PID: 2986 Comm: bash Not tainted 5.0.0 #7 > [ 88.005489] Hardware name: Renesas Salvator-X board based on r8a7795 E= S1.x (DT) > [ 88.012791] Call trace: > [ 88.015235] dump_backtrace+0x0/0x190 > [ 88.018891] show_stack+0x14/0x1c > [ 88.022204] dump_stack+0xb0/0xec > [ 88.025514] print_circular_bug.isra.32+0x1d0/0x2e0 > [ 88.030385] __lock_acquire+0x1318/0x1864 > [ 88.034388] lock_acquire+0xc4/0x22c > [ 88.037958] __kernfs_remove+0x258/0x2c4 > [ 88.041874] kernfs_remove_by_name_ns+0x50/0xa0 > [ 88.046398] remove_files.isra.1+0x38/0x78 > [ 88.050487] sysfs_remove_group+0x48/0x98 > [ 88.054490] sysfs_remove_groups+0x34/0x4c > [ 88.058580] device_remove_attrs+0x6c/0x7c > [ 88.062671] device_del+0x11c/0x33c > [ 88.066154] device_unregister+0x14/0x2c > [ 88.070070] pwmchip_sysfs_unexport+0x40/0x4c > [ 88.074421] pwmchip_remove+0xf4/0x13c > [ 88.078163] rcar_pwm_remove+0x28/0x34 > [ 88.081906] platform_drv_remove+0x24/0x64 > [ 88.085996] device_release_driver_internal+0x18c/0x21c > [ 88.091215] device_release_driver+0x14/0x1c > [ 88.095478] unbind_store+0xe0/0x124 > [ 88.099048] drv_attr_store+0x20/0x30 > [ 88.102704] sysfs_kf_write+0x54/0x64 > [ 88.106359] kernfs_fop_write+0xe4/0x1e8 > [ 88.110275] __vfs_write+0x40/0x184 > [ 88.113757] vfs_write+0xa8/0x19c > [ 88.117065] ksys_write+0x58/0xbc > [ 88.120374] __arm64_sys_write+0x18/0x20 > [ 88.124291] el0_svc_common+0xd0/0x124 > [ 88.128034] el0_svc_compat_handler+0x1c/0x24 > [ 88.132384] el0_svc_compat+0x8/0x18 >=20 > This warning occurs because pwmchip_remove still keeps pwm_lock > when removing sysfs. That's why it leads to that conflict. > Hence, this patch unlocks pwm_lock before removing sysfs. > Also, pwmchip_sysfs_export() doesn't seem to need the pwm_lock > held so that to achieve consistance between export and > unexport this patch also modifies it. >=20 > Signed-off-by: Phong Hoang > [shimoda: revise the commit log and code] > Fixes: 76abbdde2d95 ("pwm: Add sysfs interface") > Signed-off-by: Yoshihiro Shimoda > Tested-by: Hoan Nguyen An > --- > drivers/pwm/core.c | 12 ++++++++---- > 1 file changed, 8 insertions(+), 4 deletions(-) >=20 > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c > index 1581f6a..2fdd6611 100644 > --- a/drivers/pwm/core.c > +++ b/drivers/pwm/core.c > @@ -311,10 +311,12 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip, > if (IS_ENABLED(CONFIG_OF)) > of_pwmchip_add(chip); > =20 > - pwmchip_sysfs_export(chip); > - > out: > mutex_unlock(&pwm_lock); > + > + if (!ret) > + pwmchip_sysfs_export(chip); > + > return ret; > } > EXPORT_SYMBOL_GPL(pwmchip_add_with_polarity); > @@ -368,10 +370,12 @@ int pwmchip_remove(struct pwm_chip *chip) > =20 > free_pwms(chip); > =20 > - pwmchip_sysfs_unexport(chip); > - > out: > mutex_unlock(&pwm_lock); > + > + if (!ret) > + pwmchip_sysfs_unexport(chip); > + I don't exactly remember why he sysfs unexport happens this late. It's completely asymmetric to what we do in pwmchip_add_with_polarity() and commit 0733424c9ba9 ("pwm: Unexport children before chip removal") is a strong indication that this was wrong to begin with. Maybe we should just move pwmchip_sysfs_unexport() where it belongs, which is right after pwmchip_sysfs_unexport_children(). In that case, we probably do not need separate functions anymore either. We also really want to remove sysfs irrespective of whether or not the chip will be removed as a result of pwmchip_remove(). We can only assume that the driver will be gone after that, so we shouldn't leave any dangling sysfs files around. Yoshihiro, does it work if you move pwmchip_sysfs_unexport() to the top of pwmchip_remove(), right below pwmchip_sysfs_unexport_children(), instead? Does that get rid of the lockdep warning as well? That way it is also outside of the pwm_lock section, which indeed doesn't seem to be needed. Moving the pwmchip_sysfs_export() call outside of that section also seems fine and it'd be perfectly symmetric with pwmchip_remove() again. 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