[PATCH 02/15] iio: trigger: fail the attach if no sub-IRQ is left

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Josef Bacik <josef@toxicpanda.com>

A trigger can serve CONFIG_IIO_CONSUMERS_PER_TRIGGER devices, two by
default.  When one more device is attached, iio_trigger_get_irq() returns
-ENOMEM.  iio_trigger_attach_poll_func() prints an error, drops the module
reference and jumps to its exit, but ret is still 0 there, so it reports
success.

iio_enable_buffers() goes on as if the poll function were attached.  The
write to buffer/enable succeeds, but no sample ever arrives.  When the
buffer is disabled again, iio_trigger_detach_poll_func() undoes an attach
which has not taken place, with -ENOMEM in pf->irq:

 - iio_trigger_put_irq() calls clear_bit() with the bit number
   -ENOMEM - trig->subirq_base, which clears a bit in whatever lies in
   front of trig->pool;
 - if only one of the real consumers is left, the trigger is switched off
   under it;
 - module_put() drops a reference to the driver module which has been
   dropped already.

All it takes is to set the same trigger as current_trigger of three
devices and to enable their buffers.

Return the error.

Fixes: 995438233579 ("iio: Fix error handling in iio_trigger_attach_poll_func")
Cc: stable <stable@kernel.org>
Cc: David Lechner <dlechner@baylibre.com>
Cc: "Nuno Sá" <nuno.sa@analog.com>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: Leonard Crestez <cdleonard@gmail.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/iio/industrialio-trigger.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/iio/industrialio-trigger.c b/drivers/iio/industrialio-trigger.c
index 76926fee0f84..fc4651490d42 100644
--- a/drivers/iio/industrialio-trigger.c
+++ b/drivers/iio/industrialio-trigger.c
@@ -293,6 +293,7 @@ int iio_trigger_attach_poll_func(struct iio_trigger *trig,
 	if (pf->irq < 0) {
 		pr_err("Could not find an available irq for trigger %s, CONFIG_IIO_CONSUMERS_PER_TRIGGER=%d limit might be exceeded\n",
 			trig->name, CONFIG_IIO_CONSUMERS_PER_TRIGGER);
+		ret = pf->irq;
 		goto out_put_module;
 	}
 

-- 
2.56.0