Ido Schimmel
2021-Feb-10 12:21 UTC
[Bridge] [PATCH v3 net-next 00/11] Cleanup in brport flags switchdev offload for DSA
On Wed, Feb 10, 2021 at 02:01:06PM +0200, Vladimir Oltean wrote:> On Wed, Feb 10, 2021 at 01:05:57PM +0200, Nikolay Aleksandrov wrote: > > On 10/02/2021 13:01, Vladimir Oltean wrote: > > > On Wed, Feb 10, 2021 at 12:52:33PM +0200, Nikolay Aleksandrov wrote: > > >> On 10/02/2021 12:45, Vladimir Oltean wrote: > > >>> Hi Nikolay, > > >>> > > >>> On Wed, Feb 10, 2021 at 12:31:43PM +0200, Nikolay Aleksandrov wrote: > > >>>> Hi Vladimir, > > >>>> Let's take a step back for a moment and discuss the bridge unlock/lock sequences > > >>>> that come with this set. I'd really like to avoid those as they're a recipe > > >>>> for future problems. The only good way to achieve that currently is to keep > > >>>> the PRE_FLAGS call and do that in unsleepable context but move the FLAGS call > > >>>> after the flags have been changed (if they have changed obviously). That would > > >>>> make the code read much easier since we'll have all our lock/unlock sequences > > >>>> in the same code blocks and won't play games to get sleepable context. > > >>>> Please let's think and work in that direction, rather than having: > > >>>> + spin_lock_bh(&p->br->lock); > > >>>> + if (err) { > > >>>> + netdev_err(p->dev, "%s\n", extack._msg); > > >>>> + return err; > > >>>> } > > >>>> + > > >>>> > > >>>> which immediately looks like a bug even though after some code checking we can > > >>>> verify it's ok. WDYT? > > >>>> > > >>>> I plan to get rid of most of the br->lock since it's been abused for a very long > > >>>> time because it's essentially STP lock, but people have started using it for other > > >>>> things and I plan to fix that when I get more time. > > >>> > > >>> This won't make the sysfs codepath any nicer, will it? > > >>> > > >> > > >> Currently we'll have to live with a hack that checks if the flags have changed. I agree > > >> it won't be pretty, but we won't have to unlock and lock again in the middle of the > > >> called function and we'll have all our locking in the same place, easier to verify and > > >> later easier to remove. Once I get rid of most of the br->lock usage we can revisit > > >> the drop of PRE_FLAGS if it's a problem. The alternative is to change the flags, then > > >> send the switchdev notification outside of the lock and revert the flags if it doesn't > > >> go through which doesn't sound much better. > > >> I'm open to any other suggestions, but definitely would like to avoid playing locking games. > > >> Even if it means casing out flag setting from all other store_ functions for sysfs. > > > > > > By casing out flag settings you mean something like this? > > > > > > > > > #define BRPORT_ATTR(_name, _mode, _show, _store) \ > > > const struct brport_attribute brport_attr_##_name = { \ > > > .attr = {.name = __stringify(_name), \ > > > .mode = _mode }, \ > > > .show = _show, \ > > > .store_unlocked = _store, \ > > > }; > > > > > > #define BRPORT_ATTR_FLAG(_name, _mask) \ > > > static ssize_t show_##_name(struct net_bridge_port *p, char *buf) \ > > > { \ > > > return sprintf(buf, "%d\n", !!(p->flags & _mask)); \ > > > } \ > > > static int store_##_name(struct net_bridge_port *p, unsigned long v) \ > > > { \ > > > return store_flag(p, v, _mask); \ > > > } \ > > > static BRPORT_ATTR(_name, 0644, \ > > > show_##_name, store_##_name) > > > > > > static ssize_t brport_store(struct kobject *kobj, > > > struct attribute *attr, > > > const char *buf, size_t count) > > > { > > > ... > > > > > > } else if (brport_attr->store_unlocked) { > > > val = simple_strtoul(buf, &endp, 0); > > > if (endp == buf) > > > goto out_unlock; > > > ret = brport_attr->store_unlocked(p, val); > > > } > > > > > > > Yes, this can work but will need a bit more changes because of br_port_flags_change(). > > Then the netlink side can be modeled in a similar way. > > What I just don't understand is how others can get away with doing > sleepable work in atomic context but I can't make the notifier blocking > by dropping a spinlock which isn't needed there, because it looks ugly :DCan you please point to the bug? I'm not following
Vladimir Oltean
2021-Feb-10 12:29 UTC
[Bridge] [PATCH v3 net-next 00/11] Cleanup in brport flags switchdev offload for DSA
On Wed, Feb 10, 2021 at 02:21:05PM +0200, Ido Schimmel wrote:> On Wed, Feb 10, 2021 at 02:01:06PM +0200, Vladimir Oltean wrote: > > On Wed, Feb 10, 2021 at 01:05:57PM +0200, Nikolay Aleksandrov wrote: > > > On 10/02/2021 13:01, Vladimir Oltean wrote: > > > > On Wed, Feb 10, 2021 at 12:52:33PM +0200, Nikolay Aleksandrov wrote: > > > >> On 10/02/2021 12:45, Vladimir Oltean wrote: > > > >>> Hi Nikolay, > > > >>> > > > >>> On Wed, Feb 10, 2021 at 12:31:43PM +0200, Nikolay Aleksandrov wrote: > > > >>>> Hi Vladimir, > > > >>>> Let's take a step back for a moment and discuss the bridge unlock/lock sequences > > > >>>> that come with this set. I'd really like to avoid those as they're a recipe > > > >>>> for future problems. The only good way to achieve that currently is to keep > > > >>>> the PRE_FLAGS call and do that in unsleepable context but move the FLAGS call > > > >>>> after the flags have been changed (if they have changed obviously). That would > > > >>>> make the code read much easier since we'll have all our lock/unlock sequences > > > >>>> in the same code blocks and won't play games to get sleepable context. > > > >>>> Please let's think and work in that direction, rather than having: > > > >>>> + spin_lock_bh(&p->br->lock); > > > >>>> + if (err) { > > > >>>> + netdev_err(p->dev, "%s\n", extack._msg); > > > >>>> + return err; > > > >>>> } > > > >>>> + > > > >>>> > > > >>>> which immediately looks like a bug even though after some code checking we can > > > >>>> verify it's ok. WDYT? > > > >>>> > > > >>>> I plan to get rid of most of the br->lock since it's been abused for a very long > > > >>>> time because it's essentially STP lock, but people have started using it for other > > > >>>> things and I plan to fix that when I get more time. > > > >>> > > > >>> This won't make the sysfs codepath any nicer, will it? > > > >>> > > > >> > > > >> Currently we'll have to live with a hack that checks if the flags have changed. I agree > > > >> it won't be pretty, but we won't have to unlock and lock again in the middle of the > > > >> called function and we'll have all our locking in the same place, easier to verify and > > > >> later easier to remove. Once I get rid of most of the br->lock usage we can revisit > > > >> the drop of PRE_FLAGS if it's a problem. The alternative is to change the flags, then > > > >> send the switchdev notification outside of the lock and revert the flags if it doesn't > > > >> go through which doesn't sound much better. > > > >> I'm open to any other suggestions, but definitely would like to avoid playing locking games. > > > >> Even if it means casing out flag setting from all other store_ functions for sysfs. > > > > > > > > By casing out flag settings you mean something like this? > > > > > > > > > > > > #define BRPORT_ATTR(_name, _mode, _show, _store) \ > > > > const struct brport_attribute brport_attr_##_name = { \ > > > > .attr = {.name = __stringify(_name), \ > > > > .mode = _mode }, \ > > > > .show = _show, \ > > > > .store_unlocked = _store, \ > > > > }; > > > > > > > > #define BRPORT_ATTR_FLAG(_name, _mask) \ > > > > static ssize_t show_##_name(struct net_bridge_port *p, char *buf) \ > > > > { \ > > > > return sprintf(buf, "%d\n", !!(p->flags & _mask)); \ > > > > } \ > > > > static int store_##_name(struct net_bridge_port *p, unsigned long v) \ > > > > { \ > > > > return store_flag(p, v, _mask); \ > > > > } \ > > > > static BRPORT_ATTR(_name, 0644, \ > > > > show_##_name, store_##_name) > > > > > > > > static ssize_t brport_store(struct kobject *kobj, > > > > struct attribute *attr, > > > > const char *buf, size_t count) > > > > { > > > > ... > > > > > > > > } else if (brport_attr->store_unlocked) { > > > > val = simple_strtoul(buf, &endp, 0); > > > > if (endp == buf) > > > > goto out_unlock; > > > > ret = brport_attr->store_unlocked(p, val); > > > > } > > > > > > > > > > Yes, this can work but will need a bit more changes because of br_port_flags_change(). > > > Then the netlink side can be modeled in a similar way. > > > > What I just don't understand is how others can get away with doing > > sleepable work in atomic context but I can't make the notifier blocking > > by dropping a spinlock which isn't needed there, because it looks ugly :D > > Can you please point to the bug? I'm not followingFor example, mlxsw eventually calls mlxsw_sp_fid_flood_set from the SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS handling data path, and this function allocates memory with GFP_KERNEL. Another example is prestera which eventually calls prestera_fw_send_req which takes a mutex_lock. Yet another example are mv88e6xxx and b53 which use MDIO and SPI from their .port_egress_floods implementation, buses which have might_sleep() in them.